Modulator for high frequency oscillators



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E. G. LINDER MODULATOR FOR HIGH FREQUENCY OSCILLATORS Original Filed Aug. 20, 1935 2 Sheets-Sheet 2 GRNO'ELED Patented Jan. 7, 1941 UNITED STATES PATENT OFFICE MODULATOR FOR HIGH FREQUENCY OSCILLATORS Ernest G. Linder, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware 3 Claims.

My invention relates to the modulation of high frequency oscillations. More specifically, my invention applies to the modulation of magnetron oscillators.

This application is a division of Patent No. 2,139,238, which issued December 6, 1938 on my copending application Serial No. 36,973, filed August 20, 1935, and entitled "Modulator for high frequency oscillator.

One of the objects of my invention is to modulate an oscillator of the magnetron type.

Another object is to frequency modulate a magnetron oscillator.

Another object is to amplitude modulate an oscillator.

A further object is to frequency modulate and compensate for amplitude modulation and vice versa.

Other objects and advantages will appear from the following description of my invention.

I am aware of proposals to modulate a magnetron by varying its magnetic field. A description of this method of modulation may be found in United States Patent 2,005,793 which issued on June 25, 1935, to N. E. Lindenblad. The embodiments of my invention, which I am about to describe, are especially suited to the modulation of magnetrons employing a large inductive winding to develop the permanent magnetic field required for ultra high frequency oscillation. The details of my invention may be best understood by referring to the accompanying drawings and specification.

Figure I represents a magnetron oscillator embodying one form of my invention,

Figures II (a) and (b) are illustrations of the electron path in a magnetron oscillator with and without the magnetic field.

Figure III is a diagram of another embodiment of my invention,

Figure IV is a diagram of a further modification of my invention,

Figure V illustrates in schematic form an embodiment of my invention for applying frequency modulation, and

Figure VI is a diagram of my invention applied to a magnetron oscillator for amplitude modulation.

In Fig. I, the north and south poles of a magnetic field are represented by N and S. The field may be created with a conventional solenoid with or without an iron core. Within the magnetic field is a magnetron l. The cathode 3 lies in a line approximately parallel to the magnetic flux and is energized by a battery 5 or any suitable source of power. Two anodes l and 9 are symmetrically arranged about the cathode. The anodes are preferably shaped in the form of half a cylinder. The cathode lies along the central axis of the anodes. The anodes are connected together by a circuit l l which may be resonant to the desired oscillator frequency. Between the center point l3 and the cathode is an anode battery IS. The negative terminal of I5 is connected to the cathode. A pair of modulating coils I1 and I 9 are serially connected to battery 2| and microphone 23. The coils I! and [9 are positioned so that their magnetic field is substantially in alignment with the field between N and S. The coils may be connected in series or parallel but their fields must be in the same sense. The direction of their fields may either aid or oppose the field between N and S.

In Figure II (a) is a diagrammatic illustration which shows a section of a magnetron oscillator. Within the evacuated envelope 3| is a cathode 33 and the anodes 35 and 31. The radial dotted lines from the cathode to the anodes represent electron paths which are not influenced by a magnetic field. When a magnetic field of proper value, whose flux is substantially parallel to the cathode, is applied, the electron paths become arcuate as shown by the curved dotted lines in Fig. II (b). The cathode heating circuits, anode circuits, and batteries are omitted from Figures II (a) and (b) for simplicity of illustration.

Briefly, the effect of electron flow in a magnetron under the influence of the magnetic field of proper intensity is to set up high frequency oscillations. The wave length of the oscillations may be determined by the equation equals H where equals wave length in centimeters H equals magnetic field in gauss.

ultra high frequency magnetron oscillator. I have found that by employing separate modulating coils, such as I! and I9, frequency modulation may be employed in cases where it would be very impractical to modulate at voice frequencies when a single large inductive electro magnet is employed to generate the normal oscillations and the modulated oscillations.

Although the separate modulating coils may be positioned so that their magnetic fields are substantially parallel to the main magnetic field, I find cases where the modulating fields may be located at an angular position. Such an arrangement is shown in simplified form in Figs. III and IV. In these figures the coil or coils are arranged so that the flux generated by currents flowing in the modulating coils is at a right angle to the flux of the main magnetron field. In showing a right angle, I do not intend to be limited to precisely 90 because other angular arrangements may be used. The angular relationship is a decided advantage in that the action of the modulating coils is not greatly affected by the usual large iron core of the main magnet. The resulting flux is the vectoral sum of the modulating coil flux and the main magnetron coil flux. The combined fluxes produce frequency modulated oscillations.

It is often desirable to frequency modulate without any variations in the amplitude of the oscillatory currents. In the preceding description, the frequency modulation is accompanied by some amplitude modulation. The undesired amplitude modulation may be compensated by the arrangement shown in Fig. V.

In Fig. V a magnetron 4| has within its evacuated envelope, which may be of any suitable shape, two anodes 43 and 45 and cathode 41. The anodes are connected to a resonant circuit 49, which is coupled to the antenna 5|. The mid-point of the bridging conductor 48 is connected through the secondary 53, of transformer 52 and anode battery 55, to cathode 41. The negative pole of the battery 55 is connected to cathode. A tertiary winding 51 of transformer 52 is connected to potentiometer 59. Between the lower end of 59 and the slider 6| is connected an impedance 63. The mid-point of 63 is connected to one terminal of modulating coil 65. The other terminal of 65 is connected to the junction of resistance 61 and capacity 69. The resistance 61 connects to one end of 63 and the capacity 69 to the other. The elements 63-61-69 form a phase adjusting circuit. The primary 1| of transformer 52 is serially connected to battery 13 and microphone 15. It should be understood that any variable signal representing current may be impressed on II, or any of the several modulating circuits. The main magnetic field is represented by the field coil 42 and battery 44.

The current fluctuations in 'II induce changing potentials in 51. The varying potentials in 51 cause modulating currents to flow in the modulating coil 65. These current changes are accompanied by magnetic flux changes which frequency and amplitude modulate the oscillations produced in 4 I. The amplitude modulations may be offset by properly phased potential variations in 53 which applied to 43 and 45 primarily cause amplitude modulations. The amplitude modulations in 53, 43, 45 are properly phased to compensate the undesired amplitude modulations induced by 65. The exact phasing may be effected by properly adjusting the phasing circuit 61 and 69. The potentiometer 6| permits an adjustment of the amplitude modulations in 53, 43, 45

which exactly balances out the amplitude modulations induced by 65 and leaves desired frequency modulation.

In Fig. V1 is illustrated the embodiment of my invention which may be used to amplitude modulate with compensations for any accompanying frequency modulation. A microphone 8| is serially connected through battery 83 to the primary 85 of transformer 81. The mid-tapped secondary 89 has its mid-tap connected through rheostat 9| to cathode 93 of the magnetron 95. The anodes 91 and 99 are connected to the resonant circuit IM and antenna I03. The mid-point of IN is connected to the positive terminal of battery I05. The negative terminal of I05 is connected to resistance I 01 and capacity I09. The resistance I01 and capacity I09 are also connected to the outer terminals of 89. The elements I01 and I09 form a phase correcting circuit. Potential variations in 89 cause amplitude modulations of the high frequency oscillations generated by 95 and its circuits. (The normal magnetron field is omitted for simplicity of illustration.)

The transformer 61 has a tertiary winding Ill. Across III is shunted a potentiometer H3. A connection is made from the junction of II I and H3 to one terminal of the ionic modulator H5. The other terminal oi. H5 is connected to the biasing battery I I1 and the slidable connection on I I3. The ionic modulator may be of the general type disclosed in the Proceedings of the Institute of Radio Engineers of June 1934, volume 22, No. 6, pages 791-793, Note on an ionized gas modulator for short waves by Linder and Wolff. It has been found that a. gas changes its absorption and index of refraction for electromagnetic waves when its degree of ionization is changed. It is this phenomena which is applied in the present invention to modulate the waves which are radiated through the ionlzable gas contained within the ionic modulator I I5.

By suitably adjusting the phases and relative amplitudes of the two modulating effects, the resulting modulation will be of the amplitude type with the frequency modulations substantially cancelled.

The circuit shown in Fig. V is the preferred arrangement for frequency modulation. The same circuit may be adjusted for amplitude modulation with the frequency modulation efiects cancelling each other. Likewise, the circuit of Fig. 6 may be employed to frequency modulate with amplitude modulations balanced out. It is within the scope of those skilled in the art to arrange other combinations of the arrangements I have shown such as, ionic modulation with magnetic modulation for either frequency or amplitude modulation.

The foregoing description illustrates one embodiment of my invention which may be employed to amplitude or frequency modulate a magnetron oscillation. Other modifications within the scope of my invention will occur to those skilled in the art. I do not intend to limit my invention to the precise arrangement shown except as required by the prior art and the appended claims.

I claim as my invention:

1. In a system of the character described, a magnetron oscillator, a resonant circuit connected to said oscillator, an antenna, means coupling said antenna to said resonant circuit, an ionic modulator in the path of oscillations radiatcd from said antenna for varying the ampli- 7 tude and frequency 01' said radiated oscillations, means for modulating the amplitude and frequency of oscillations generated by said magnetron, and phase adjusting means for balancing out one type of modulation without eliminating the other.

2. In a system of the character described, a magnetron oscillator, an oscillatory circuit connected to said oscillator, an antenna, means coupling said oscillatory circuit and said antenna, an ionic modulator in the path of radiations from said antenna, a source of variable signal representing currents, means for applying said currents to said magnetron oscillator so that said currents modulate the frequency and amplitude of oscillations generated by said magnetron, means for applying said currents to said ionic modulator to modulate the frequency and amplitude of the radiations from said antenna, and means for adjusting the phase of modulations produced by one of said means so that said frequency modulations are minimized.

3. The method of modulating a magnetron os cillator which comprises generating oscillatory currents, modulating the frequency and amplitude of said currents, radiating said currents, passing said radiated currents through an ionizable region, varying the ionization of said region in responsive to signal representing voltages to modulate the frequency and amplitude of said radiated currents, and adjusting the relative phase of the modulation of said oscillatory currents and said radiated currents to minimize said frequency modulations.

ERNEST G. LINDER. 

